The focus of this program project is to assess physical and behavioral attributes of autistic individuals and their relatives in order to resolve etiologic heterogeneity and to better characterize variable expression. Four projects are proposed which are integrated by the common themes of extended phenotype characterization and identification of etiologic subgroups. Project I will test the hypothesis that immunologic abnormalities may be involved in the pathogenesis of autism in a subgroup of individuals. Variation in the major histocompatibility complex (MHC) will be evaluated in autistic individuals, family members, and controls. This variation will in turn be related to IgA deficiency and serotonin levels. Project II will test the hypothesis that relatives of autistic individuals exhibit a subtle cognitive variant of autism. Executive function will be assessed in autistic individuals, relatives, and controls to characterize more completely the variation in expression of autism. Project III will attempt to extend the phenotype of autism by assessing autistic subjects, family members, and controls for other psychiatric abnormalities. It will test the hypotheses that subgroups of autism can be defined by the presence of minor physical anomalies and/or by abnormalities in retinal function. Project IV will test the hypothesis that increased brain or head size defines a subgroup of autistic individuals. Brain volume and pathology will be assessed by magnetic resonance imaging in samples of autistic individuals and compared to non-autistic controls with normal or large head size. These projects are supported by an administrative office, a biostatistics service, and an assessment facility.